Flux ropes in space plasmas

Alexey Isavnin Supervisors: Emilia Kilpua, Hannu Koskinen

University of Helsinki, Finland

Graduate seminar series 2014, March 24

Outline

•  Space weather: Sun–Earth connection, its mechanism and effect on us

•  Coronal mass ejections: multipart configuration and embedded flux ropes

•  Evolution of solar flux ropes: deflections and rotations

•  Magnetospheric flux ropes: evolution and substorm dynamics

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Space weather

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Space weather describes the conditions in space that affect Earth and its technological systems

Space weather. Spectacular…

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Space weather. Spectacular…

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Space weather. Spectacular but hazardous

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Space weather. How does it work

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Coronal mass ejections (CMEs) are the drivers of the strongest magnetospheric storms. Geoffective CME is the one that caused geomagnetic disturbance.

Space weather. How does it work

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Flux rope CMEs and their internal structure

Magnetic flux ropes

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•  Local cylindrical geometry •  Helical magnetic field lines with zero twist in the core and

increasing with the distance from the axis •  Maximum magnetic field strength along the axis

Coronal mass ejection

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CMEs are not just explosions on the Sun but eruptions of magnetic flux ropes.

Flux rope formation

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Flux rope forms prior to CME. Flux rope eruption happens in conjunction with solar flare.

CME observations

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CME can be observed in white-light or EUV from several viewpoints and in situ by a spacecraft it encountered. A flux rope measured in situ is known as magnetic cloud (MC).

Five-part CME structure

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The dark cavity represents the flux rope. Bright core is the prominence material. Faint loop is the signature of a shock wave driven by the CME.

Five-part ICME structure

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The in-situ measured interplanetary CME (ICME) is a multipart structure too. MC is only a part of it.

Five-part ICME structure

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Front, rear and MC regions consist from physically different plasma, i.e. originate from different physical processes or regions near the Sun.

Conclusions

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•  CMEs and ICMEs are both multipart structures with five distinct parts distinguishable.

•  Flux rope occupies the dark cavity area of a CME observable in white light.

•  Front and rear MC parts originate near the Sun and correspond to piled-up material (bright loop) in front of the flux rope and prominence material (bright core), respectively.

•  Sheath region region form during fast CME propagation and occupies the region of diffusive emission.

Evolution of solar flux ropes

Flux rope evolution

•  Expansion •  Longitudinal deflection •  Latitudinal deflection •  Rotation •  Distortion

Motivation: Change of flux rope orientation can result in change of geomagnetic effectiveness. Important for space weather forecasting.

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Tracking a flux rope requires several tools

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solar disk observations coronagraph imaging in-situ measurements

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Eruptive prominence

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Post-eruption arcades or eruptive prominences give idea about geometrical orientation of the flux rope in the lower corona.

Flux rope signatures in the lower corona

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Coronagraph observations of flux ropes

Forward modeling of ejected flux ropes gives an estimate of their orientation in the inner heliosphere.

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In-situ measurements as a constraint

Local orientation of the flux rope invariant axis is only a constraint for its global orientation.

Magnetic field map by Grad-Shafranov reconstruction

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Flux rope propagation through MHD solar wind

We propagate the flux rope in 3D through MHD-simulated solar wind using in-situ measurements as a constraint.

Longitudinal deflection

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Latitudinal deflection is caused by the magnetic interaction with the Parker-spiral-structured solar wind.

Deflection towards equatorial plane

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Flux rope global axis direction during its travel from the Sun to 1 AU.

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Rotation relative to heliospheric current sheet

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Flux rope orientation superimposed on velocity (top) and magnetic energy density (bottom) maps at 1 AU for two events.

Conclusions

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•  Flux ropes continuously deflect towards the solar equatorial plane during their travel from the Sun to 1 AU.

•  Flux ropes rotate while getting approximately aligned with heliospheric current sheet.

•  Geometrical evolution of ejected flux ropes in the inner heliosphere was found to be caused by magnetic interaction with Parker-spiral-structured solar wind.

•  60% of flux evolution happens during the first 14% of their travel distance from the Sun to 1 AU.

Evolution of magnetospheric flux ropes

Magnetospheric substorm dynamics

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1.  Energy from the solar wind due to interaction with magnetic structures within is stored as excess magnetic flux in the magnetosphere.

2.  A reconnection site (X-line) is formed in the magnetotail. 3.  During the explosive substorm reconnection part of excess

energy is released tailwards and part is dissipated in the ionosphere increasing auroral luminosity.

Plasmoid formation

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Plasmoid is a flux-rope-like structure formed between N2 and N3 X-lines. It carries away the excess energy from the magnetosphere.

Multiple X-line reconnection

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Due to plasma instabilities multiple X-lines can be dynamically generated at the near-Earth reconnection site. Flux ropes formed in between the X-lines can be released both tailwards and Earthwards.

Sequential tailward flux ropes

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An example of a chain of six flux ropes released sequentially tailwards during just 45 minutes. The sixth flux rope had a larger tilt, speed, core field and size, and corresponded to the change of solar wind conditions and formation of new reconnection site.

Earthward moving flux ropes

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Earthward moving flux ropes are often registered in far tail and very rarely in the near tail. The reason is continuous deterioration due to anti-reconnection process.

Conclusions

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•  Multi-X-line sites are dynamic regions and result from plasma instabilities. Flux ropes can be formed and ejected sequentially from these areas both tailwards and Earthwards.

•  The properties of released flux ropes reflect solar wind conditions and their change correspond to reconfiguration of the magnetosphere.

•  Earthward moving flux rope get deteriorated due to anti-reconnection and eventually degrade into dipolarization fronts.

Thanks for your attention!